(1) Field of the Invention
The present invention relates to a radio base station that communicates with a plurality of mobile stations with a space division multiplex, and relates to a mobile station.
(2) Description of Related Art
Recently, digital communication apparatuses transfer information by modulating a carrier with a digital information signal (baseband signal) to efficiently transfer data.
In digital communications, a multiple channel system is adopted to assign a plurality of users to the same frequency to efficiently use frequencies. This is achieved by improvements in the data transfer speed or development of the time division multiplex. In addition to the above techniques, a technique called space division multiplex is currently receiving attention. The space division multiplex enables a plurality of channels to be assigned to the same frequency at the same time using the adaptive array method.
In the adaptive array method, a radio wave is transmitted or received to/from a remote end in a certain direction by adaptively forming a directional pattern (also referred to as an array antenna pattern) using a plurality of antennas. The adaptive array method is described in detail in “Transactions on Adaptive Signal Processing in Space Domain and Applied Techniques” (The Transactions of the Institute of Electronics and Communication, Vol.J75-B-II, No.11, Nov.), and will not be detailed here.
In the space division multiplex, an adaptive-array radio base station communicates with a plurality of mobile stations simultaneously by forming a different directional pattern for each mobile station on each carrier of the same frequency simultaneously. The space division multiplex is also called Path Division Multiple Access (PDMA). The PDMA is described in detail in “PDMA Mobile Communication System” (Technical Report of Institute of Electronics, Information and Communication Engineers, RCS93-84(1994-01), pp37-44), and will not be detailed here.
When the space division multiplex is applied to a mobile communication system such as a portable telephone or a PHS (Personal Handyphone System) telephone, the radio base station needs to form an array antenna pattern for either of transmission and reception. This is because it is physically difficult for a mobile station to have an array antenna and because the mobile station's position and direction greatly changes due to its portability.
Meanwhile, many of recent portable telephones or PHS telephones have an embedded antenna in addition to a whip antenna to perform a diversity reception. It should be noted here that the diversity reception is divided into (1) the antenna selection diversity in which one out of a plurality of antennas that provides a reception signal with the highest reception level is selected and the signal is received through the selected antenna, and (2) the maximal-ratio combiner diversity in which received signals are weighted in proportion to each reception level and combined, and the combined signal is received.
However, there is a problem that when a space-division-multiplex base station uses the space division multiplex to communicate with a plurality of mobile stations performing the diversity reception, the communication quality deteriorates. The reason for this is thought that a positional difference between the whip antenna and the embedded antenna greatly affects reception of the array antenna pattern. More specifically, since many mobile stations use a whip antenna for transmission, the radio base station often points the array antenna pattern at a spatial position of the whip antenna. In this case, if the embedded antenna is located at such a position as is interfered with an array antenna pattern for another mobile station, the embedded antenna may be selected by mistake since the diversity reception selects an antenna based on only the reception signal level. This leads to deterioration of the communication quality (e.g., increase in the bit error rate).